First report of cattle farms with gastrointestinal nematodes resistant to levamisole in Mexico.

The objectives of the present study were: (1) to report the percentage of cattle farms with gastrointestinal nematodes (GINs) resistant to levamisole in Veracruz, Mexico, (2) to identify the genera of GINs involved in resistance, and (3) to identify factors associated with these resistances. The faecal egg count reduction test (McMaster technique) was used to detect the presence of resistant GINs. A questionnaire was given to owners to understand the history of anthelmintic use. The percentage of cattle farms with GINs resistant to levamisole was 36.4% (4/11). The percentage of faecal egg count reduction on resistant farms was 91%, 82%, 42% and 88%. A similar number of cattle farms (4/11) were identified as potentially having levamisole resistance. Only three farms had GIN populations susceptible to levamisole. Cooperia spp. was the genus most commonly found to be resistant, followed by Haemonchus spp., Ostertagia spp. and Oesophagostomum spp. No factors were identified that influenced the presence of GIN resistance. However, there were identified inappropriate anthelmintic practices in cattle farms that should be improved. None of the farmers weighed their animals in order to dose them correctly with anthelmintics. Six cattle farms (54.5%) applied anthelmintics to new arriving animals. This is the first report of levamisole resistant GINs in Mexico. Improving the use of anthelmintics and measures of quarantine for infected cattle will help control the spread of resistance.